Fig. REF  shows the sum-rates achieved by all setups versus the total number of reflecting elements, \(N\) . It is observed that the sum-rates achieved by the setups with integrated IRSs monotonically increase with \(N\)  due to higher passive array reflection gain. It is also observed that the “double-reflection only” setup performs closely to “single- and double- reflection” setup, and achieves a higher performance gain over the “single-reflection only” setup. The performance gap between the “double-reflection only” and “single-reflection only” setups increases with \(N\) . This is because in our proposed integrated IRS-BS architecture, the path loss of the double-reflection channel components is not severe due to the short distance between different IRSs. Moreover, the passive array reflection gain provided by the double-reflection channel components is larger than that provided by the single-reflection channel components [1]}, [2]}, and the number of double-reflection components is much larger than that of single-reflection components, i.e., \(\sum _{j=1}^{J}\sum _{q\ne j}^{J}N_{j}N_{q} \gg N\) . As a result, the double-reflection channel components have a more dominant impact on the sum-rate performance as compared to the single-reflection channel components. Furthermore, despite a limited number of IRS reflecting elements, the “single- and double- reflection” setup significantly outperforms the “no-IRS” benchmark scheme employing the same number of antennas (i.e., \(M=16\) ) or even more antennas (i.e., \(M=64\) ). Thus, the proposed integrated IRS-BS architecture is an efficient solution for enhancing the communication rate performance or reducing the number of antennas at the BSs for achieving the same rate performance.
